The FM (Frequency Modulated) screening is used in, for example, inkjet-type image recording, and gradation (tone) is expressed by changing the density of distribution of microdots, each of which is a minimum pixel unit, being irregularly arranged. The arrangement of the microdots is performed in accordance with halftone dot image data which is generated from an original image in advance. In the inkjet-type image recording, by ejecting ink onto writing positions on a recording object, the microdots are formed at the writing positions.
In order to halftone an original image of multi-gradation. i.e. of continuous tone, a threshold matrix in which a plurality of elements are arranged both in a row direction and in a column direction and respective threshold values are given to the elements is generated in advance and prepared, and the original image is compared with the threshold matrix. As a technique for generating a threshold matrix, for example, in a halftone screen generation method disclosed in Japanese Patent Application Laid Open Gazette No. 2006-94566 (Document 1), a weighting filter is operated around a position of a determined threshold value and a position of a next threshold value is sequentially determined on the basis of a distribution of cumulative weighting values.
Further, in a threshold matrix generation method disclosed in Japanese Patent Application Laid-Open No. 2008-294702 (Document 2), in a matrix area, a plurality of linear areas extending in a direction inclined with respect to a row direction and a column direction are set in advance. The plurality of linear areas are further allocated alternately to a first element group and a second element group. In each of the element groups, after one element is specified, by repeating a process of specifying an element which is farthest away from specified elements, the order of illuminating (turning on) dots is determined and a threshold value of each element is determined in accordance with this order.
On the other hand. Japanese Patent Application Laid-Open No. 2009-61727 (Document 3) discloses a technique to perform printing at double speed by using an image shrunken to a half in a width direction which is a row direction of a pixel array. In double-speed writing, by extending the shrunken image in the width direction and writing dots in a checkerboard-like manner both in the row direction and in a column direction among writing positions in normal writing, the image is recorded with suppression of deterioration in the quality of the image.
In a case where an image is recorded by using the technique disclosed in Document 3, when the image is recorded by performing halftoning on the image shrunken to a half in the width direction and extending the thus-obtained halftone dot image in the width direction, unintended deviation of writing dots sometimes occurs.